Multiple treatment modalities are available for treating cancer. They include surgical resection of the tumor, chemotherapy, radio therapies and combination therapies. While resection can be curative, tumors recur in most cases. Chemotherapies utilize drugs that kill tumor cells by intercalation of DNA, inhibition of replication, or prevention of microtubule assembly. To avoid killing healthy cells, a balance must be achieved by fine tuning the chemotherapy doses and regimens, which should be based on the type of tumor, stage, grade and overall tumor burden. Radiotherapy is essentially geared to kill cancer cells by damaging DNA. Both chemo and radiotherapies result in a number of side effects in patients, including resistance to therapies, and in most cases, tumors recur.
Conventional treatments for solid and circulating and liquid cancers typically include chemotherapy and/or surgery. Recently there has been interest in developing vaccines in an effort to stimulate an immune defense. It is believed that vaccination against tumors may result in protection from tumor recurrence due to immunological memory.
Recently, interest in immunotherapies based on cancer vaccines has prompted attempts to develop such vaccines as an additional treatment option for cancer patients. Several types of cancer vaccines have been considered, including whole cell, defined tumor antigens, peptides and DNA vaccines. Except for whole cell vaccines, all other vaccines require a thorough understanding of the tumor antigen(s) involved. The non-whole cell vaccines, such as peptide or whole protein vaccines or antigen-specific DNA vaccines require a substantial knowledge of the expression of those tumor antigens, and their immunogenicity in cancer patients. Acquiring such data involves substantial investment in defining the tumor antigens. Besides, most of these types of vaccines involve usage of one tumor antigen (e.g., telomerase antigen, prostatic acid phosphatase). The anti-tumor immunity is complex, and multiple antigens and multiple epitopes are likely to be involved for efficacious clinical outcome. In almost all cancers except the ones induced by viruses, the tumor antigens are mostly self-proteins which have been tolerized during the development of the immune system, and hence it is difficult to induce an immune response against them. In the absence of a clear cut understanding of all tumor antigens involved in breaking self-tolerance and in the induction of clinically relevant immunity against cancer tissue, whole cell vaccines become good candidates for presenting a plethora of tumor antigens to the immune system, thereby hedging against tolerogenic epitopes.
Several cancer vaccines have been tested in clinical trials, either alone or in conjunction with various adjuvants. Unfortunately, the clinical efficacy has thus far not been impressive. Nevertheless, with respect to defined vs non-defined (whole cell) vaccines, the whole cell vaccines appear to be superior.
Tumor antigen vaccines generally show poor antigenicity due to immune tolerance. Most require interventional therapies in order to provide an adequate “danger” signal to the immune system in order to activate a robust, clinically meaningful antitumor immunity.
The adaptive immune response to tumors alone is poor, mostly because the target antigens are self-proteins, except in cases of tumor induced by viruses. Since the immune system has evolved to recognize microbial/pathogenic organisms, it is possible that when self-antigens are presented by antigen presenting cells (APCs), there is normally no “danger” signal, and the APCs therefore provide tolerizing signals to avoid autoimmunity. In the presence of microbial antigens, such as Emm protein, which when expressed in tumor cells, processed and presented by the APCs, is perceived by the adaptive immune system as a “danger” signal. Mature APCs present antigens to T cells, which generate activated MHC class I restricted CD8+ and class II restricted CD4+ cells and results in a clinically effective anti-tumor immunity.
Several similar types of cancer vaccines have been tested in pre-clinical and clinical studies, but thus far only one cancer vaccine, Provenge® (Sipuleucel T, Dendreon Corporation), has been approved by the FDA and only for a single indication; i.e., for use in asymptomatic or minimally symptomatic castration resistant prostate cancer patients.
Provenge® is manufactured by “feeding” patient's APCs in vitro with prostatic acid phosphatase fused to GM-CSF (adjuvant) to induce maturation of APCs, which then stimulate the immune system after infusion into the patient. Provenge® is antigen specific and cannot be employed universally against other types of cancers. The survival benefit with Provenge® averages 4.1 months (Kantoff P W et al., N Engl J Med 2010; 363:411-22).
Autologous whole cell vaccines have been reported, including GVAX vaccine for colorectal cancer. Isolated autologous tumor cells were mixed with Bacillus Calmette-Guerin (BCG) as the adjuvant and administered to cancer patients. While some limited clinical activity was observed, it did not reach statistical significance. There was no difference in time to relapse or overall survival (OS). The ulceration induced at the vaccine site by BCG was a substantial issue. Additionally, the FDA requirement for a sterility test could not be fulfilled due to the nature of the tissue leading to premature termination of a Phase III trial.
Some success has been reported with an autologous whole cell vaccine against canine lymphoma (U.S. Pat. No. 7,795,020). Cancer cells from a solid lymphoma tumor were isolated, transformed in vitro with a vector expressing Emm55 protein on the lymphoma cell surface, and administered to the subject from whom the lymphoma cells were isolated.
While whole cell vaccines have been demonstrated to have clinical activity, manufacturing this type of vaccine requires surgical removal of patient's palpable lymph node, ex vivo processing of cells, transformation of cells with Emm protein, irradiation of tumor cells, and the quality control (QC) of the vaccine for individual patients. With solid tumors, it may be difficult to obtain sufficient cells for processing, and processing can require from one to several weeks.
Autologous melanoma and renal cell carcinoma (RCC) in combination with BCG have been tested in patients (Baars A et al., Ann Oncol (2000) 11:965-970; de Gurijil et al., Cancer Immunol Immunother (2008) 57:1569-1577). A slight improvement in 5 year OS of 33% vs 35% for melanoma and 77% vs 68% for RCC over the historic control, respectively, were observed. Unfortunately, due to ulceration at the vaccination site, BCG was disqualified. Additional trials were conducted with a BCG replacement, hypo-methylated bacterial CpG DNA, in an attempt to prove equivalency with BCG in RCC patients. With a 20% clinical response rate, the authors concluded that equivalency was demonstrated with CpG.
Regardless of some positive results, the potential side effects of CpG caused concern. The safety profile of CpG was investigated in rodents, nonhuman primates and humans. Safety issues included the possibility that CpG might increase host susceptibility to autoimmune disease or predispose to toxic shock. The immune stimulation elicited by CpG motifs can reduce the apoptotic death of stimulated lymphocytes, induce polyclonal B-cell activation and increase the production of autoantibodies and proinflammatory cytokines, all of which are known to increase the risk of autoimmune disease, especially in organ-specific autoimmune diseases. The organ-specific autoimmune diseases are typically promoted by the type of Th1 response preferentially elicited by CpG. For example, in an IL-12-dependent model of experimental allergic encephalomyelitis (that mimics multiple sclerosis), animals treated with CpG and then challenged with autoantigen developed autoreactive Th1 effector cells that caused disease, whereas mice injected with autoantigen alone remained disease free.
A more classic vaccination approach was based on stimulation of the immune system by administering an immunogenic foreign protein. In a molecular mimicry model, CpG was co-administered with Chlamydia-derived antigen. Unfortunately, this promoted the induction of autoimmune myocarditis (Bachmaier K et al., Science, 1999; 283:1335-1339). CpG also increased the susceptibility of mice to interventions that can induce arthritis. These results indicated that CpG adjuvant promotes the development of deleterious autoimmune reactions under certain circumstances. This concern was heightened when a clinical trial using CpG as an adjuvant for the hepatitis vaccine was halted after one subject developed Wegener's granulomatosis, an autoimmune disease characterized by inflammation of the vasculature. In addition to the risk of autoimmunity, several studies noted an elevation in the frequency and/or severity of local adverse events (injection site reactions such as pain, swelling, induration, pruritus and erythema) and systemic symptoms (including flu-like symptoms) by CpG-adjuvanated vaccines.
Another approach to cancer vaccines has been to mix cell lines of a selected type of cancer derived from different individuals of the same cancer. An allogeneic pancreatic cancer cell line expressing the adjuvant GM-CSF was used in a phase I clinical trial following pancreaticoduodenoectomy. The administered cells proved to be non-toxic but delayed type hypersensitivity (DTH) reaction in the recipients was observed (Jaffe, E M et al., J. Clin. Oncol. 2001, 19, 145-156).
In other studies using allogeneic cells, a prostate cancer trial employed a mixture of 3 different allogeneic prostate cancer cell lines with BCG. Time to progression improved from 28 to 58 weeks. With prostate GVAX allogeneic vaccine (a mixture of LNCaP and PC-3 cell lines expressing GM-CSF), two clinical trials were conducted—VITAL-1 and VITAL-2. In VITAL 1 trail in hormone resistant prostate cancer patients that compared GVAX against docetaxel, no difference between the groups found. The VITAL 2 trial that compared GVAX+docetaxel against docetaxel had to be terminated due to safety concerns. The adjuvant GM-CSF, an FDA approved drug capable of stimulating white blood cell growth, has been widely used in cancer vaccine trials with varying results.
DNA vaccines have been the subject of a few limited studies. Plasmid DNA vaccines are circular DNA encoding one or more tumor associated antigens (TAAs) and immune-stimulating or co-stimulating molecules, which are administered intramuscularly, intranodally or intratumorally. The local tissue specific cells and the APCs at the injection site then express the antigens to stimulate the immune system. It is believed that the cross-presentation of antigens by the APCs will be the most important factor in the induction of robust anti-tumor response with DNA vaccines. While safety and efficacy of naked plasmid vaccines have been tested in only a small number of clinical settings, at least the intramuscular, intranodal or intratumoral vaccinations have been shown to be safe and capable of eliciting immune responses to some extent in a few patients.
Naked plasmid DNA vaccines used in clinical settings include constructs coding for TAAs. For example, in cohorts of: 1) B-cell lymphoma patients-TAA: idiotypic determinants; 2) melanoma patients TAAs: gp100, MART-1-derived peptides and tyrosinase or tyrosinase-derived peptides; 3) colorectal carcinoma patients-TAA: carcinoembryonic antigen and CEA; 4) HPV-16+ cervical intraepithelial neoplasia (CIN) patients-TAA: HPV-16 E6; and 5) individuals affected by prostate carcinoma-TAA: prostate specific antigen (PSA) have been tested or are being studied. While the results of these trials are mostly not yet available, they are geared towards specific cancers, and cannot be used in multiple tumors because they depend on TAAs.